Back when I still wrote my series on Common Usability Terms, I also wrote one about the menu. The menu is a core element of the graphical user interface developed by Xerox, and it has been adopted by virtually all popular computing environments ever since. Even in this supposedly post-PC world, the menu is still very much alive and kicking - they may have remarketed them as pop-overs or "touch selection experiences" or whatever, but don't be fooled by that nonsense. A menu is a menu.
Shuttleworth seems to believe it's time to retire the concept entirely - or at least offer an alternative. Menus are awesome in that they're generally in the same place, there's a fair level of consistency (even cross-platform), and, well, they're familiar. They have downsides, too, though; it's hard to find something within menus and nested menus have a tendency to close accidentally.
The HUD does things differently. As complicated as Shuttleworth's long blog post makes it out to be, it's just a search field which only searches within menu items of one application. That's it. In all honesty, it's actually quite brilliant, and I'd love to have it on my applications right now.
The big issue that I see, however, is that it doesn't look like a replacement just yet. Only when you specifically know what command you want to invoke is the HUD useful; otherwise, it offers no discoverability features. In that sense, it's the exact opposite of Microsoft's ribbon. Luckily for us, the HUD will supplement the traditional menu at first, but Shuttleworth does plan to abolish the menu altogether eventually
They're still working on the HUD, and Shuttleworth acknowledges the discoverability issue - they're still working on a solution. I'm curious where they're going to take this idea, but I'm happy they're trying.